The present invention relates to wireless transmission systems. More particularly, the invention relates to improved frequency hopping spread spectrum systems.
Spread spectrum transmission techniques are commonly used in wireless systems. Generally, such systems employ xe2x80x9cdirect sequencexe2x80x9d or xe2x80x9cfrequency hoppingxe2x80x9d methods. In frequency hopping systems, both the transmitter and receiver operate on frequencies that change over sequential time periods according to a predetermined hop sequence. Once the receiver begins to receive transmitted data, the receiver and the transmitter independently follow the hop sequence. For example, assume that a transmitter transmits in 100 millisecond periods at each hop frequency and that the receiver monitors all frequencies within the transmitter""s operative frequency range. As the transmitter moves from frequency to frequency, the receiver tunes to those frequencies in step with the transmitter to receive the entire message. The message may be short enough, however, to require only one period.
Initially, the receiver scans or otherwise monitors the transmitter""s frequency range looking for a transmitted signal. Upon detecting a signal, the receiver determines the signal""s position in the hop sequence and synchronizes to the transmitter. Various techniques for identifying the hop sequence position should be familiar to those skilled in this art and are therefore not discussed in detail herein. Briefly, however, where the transmitter transmits at any given channel only once during the sequence, the frequency value identifies the sequence position. Thus, the receiver might determine the hop sequence position simply by identifying the channel on which the signal was transmitted.
The receiver must synchronize itself to the transmitter so that the receiver and transmitter xe2x80x9chopxe2x80x9d at the same time. If the receiver detects the initial transmission midway through the dwell period, it must therefore find the period""s end to hop to the next frequency in time for the next transmission. It may identify the period""s end simply by the end of the transmission. If the receiver then moves to the next hop frequency, it is thereafter synchronized with the transmitter. Those skilled in this art should be familiar with synchronizing techniques, which are therefore not discussed in detail herein.
Since the receiver knows all the transmission frequencies in the sequence and the order in which they are used, it is then able to monitor each subsequent frequency in the sequence to receive subsequent data signals until the message is complete. Such a system requires, however, that the transmitter and receiver be relatively accurate. If the transmitter drifts, such that the frequency upon which a given data signal is actually transmitted is offset from the intended frequency, the receiver may not be able to adequately receive it. The problem commonly occurs when the drift occurs at most or all transmission frequencies throughout the hop sequence. A similar problem exists when the receiver drifts and is compounded when both devices drift.
The present invention recognizes and addresses the foregoing disadvantages, and others, of prior art constructions and methods.
Accordingly, it is an object of the present invention to provide an improved frequency hopping spread spectrum wireless transmission system.
More particularly, it is an object of the present invention to provide such a system that is able to compensate for frequency drift in the transmitter and/or the receiver.
Some of these objects are achieved by a frequency hopping spread spectrum wireless transmission system having a transmitter and a receiver system. The transmitter is configured to transmit radio frequency data signals in sequential time periods on respective transmit channels according to a predetermined channel sequence. The data signal in each time period includes information relating the data signal to its position within the predetermined sequence. The receiver system is configured to monitor a plurality of receive channels over a frequency range that includes the transmit channels and to receive the data signals on the receive channels. The receiver system is also configured to determine, upon reception of a first data signal on a first receive channel, a next receive channel based on the first receive channel so that the next receive channel differs from the first receive channel an amount equal to the interval between the data signal""s position and the next position in the sequence, and to monitor the next receive channel for a next data signal.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the invention and, together with the description, serve to explain the principles of the invention.